An example of a conventional MEMS resonator is described with reference to FIGS. 7 and 8. FIGS. 7 and 8 are a perspective view and a cross-sectional view showing a structure of the MEMS resonator as disclosed in Patent Literature 1, which is manufactured using a SOI (Silicon on insulator) substrate. This MEMS resonator is used in, for example, a filter, as described in Patent Literature 1. Here, the SOI substrate is a substrate manufactured by forming a device-forming layer of a single crystal silicon on a silicon substrate with a BOX layer of a silicon oxide film (Buried silicon oxide film) interposed therebetween.
In the manufacturing of the MEMS resonator shown in FIG. 7, anisotropic etching is firstly conducted in the SOI substrate to form a beam-like body having a triangular section (a beam of triangular section), the silicon oxide film for forming the gap is formed, and then an electrode 202 is formed. Subsequently, the silicon oxide for the gap and the BOX layer 206 are removed leaving a portion which is to be a support portion. Thereby, an aerial protruded structure portion is completed wherein the beam of the triangular section which is to be an oscillator 201 is released so that it is in a movable state and the electrodes having space (cavity) and narrow gaps are disposed on the side surfaces of the beam of the triangular section having a protruded structure.
As shown in FIG. 8, the space (cavity) 207 is formed under the oscillator 201. This manufacturing method achieves the MEMS resonator having the oscillator of the single crystal silicon wherein the SOI substrate is employed and the electrode terminals which enable electrostatic excitation and electrostatic detection. Since both of the film for forming the gap and the BOX layer 206 which corresponds to a lower layer portion underlying the oscillator 201 are the silicon oxide layer in this manufacturing method, the gap formation and the release of structure are made simultaneously in the final release (structure release) step, resulting in the decrease in the number of the manufacturing steps. Patent Literature 1 disclose a method of covering this resonator with a glass cap as a method for sealing this resonator.
A vibration-type pressure sensor having a conventional sealing structure is described with reference to FIG. 9. FIG. 9 is a cross-sectional view of the pressure sensor which is manufactured using a MEMS technique as described in Patent Literature 2. An oscillator 103 is a beam of single crystal silicon. A vacuum chamber 105 is formed by a sacrificial-layer etching technique which employs difference in etching rate, which difference caused by an impurity content in epitaxial grown silicon.
A shell 104 is also formed by a thin-film formation technique. Electrostatic capacitance is formed between the shell 104 and the beam (oscillator) 103. The beam is anchored to a measuring diaphragm at both ends and can be vibrated at around a resonant frequency. The device shown in FIG. 9 functions as the pressure sensor by catching change of stress in the beam due to the pressure applied to the measuring diaphragm as the change in resonant frequency.
A Q value representing sharpness of resonance of the beam deteriorates due to viscosity of the air around the beam. Therefore, a high Q value can be maintained by keeping the vacuum in the vacuum chamber. As the Q value is higher, the change in resonant frequency due to the pressure can be sensed more sensitively.
The pressure sensor described with reference to FIG. 9 can be manufactured by a method wherein the resonant beam is sealed in vacuum only by the thin-film formation processes. Thus, the manufacturing of this pressure sensor does no need the vacuum sealing step in a device-packaging process, enabling a small-sized pressure sensor to be provided at a low cost.